High carbon steel sheets used for tools, automobile parts (gears and transmissions and the like are subjected to heat treatment such as quenching and tempering after punching and forming. Aiming at cost reduction, manufactures of tools and parts, or the users of high carbon steel sheets, study in recent years the simplification of conventional parts-working by machining and hot forging of cast to shift toward the press forming (including cold-forging) of steel sheets. Responding to the movement, the high carbon steel sheets as the base material are requested to have excellent ductility for forming into complex shapes and to have excellent bore expanding workability (burring property) in the forming step after punching. The bore expanding workability is generally evaluated by the stretch flangeability. Accordingly, there is wanted a material that has both excellent ductility and excellent stretch flangeability. In addition, from the point of reducing load on press machine and mold, the material is also strongly requested to be mild.
In the current state, there are studied several technologies for softening the high carbon steel sheets. For example, Japanese Patent Laid-Open No. 9-157758 proposes a method for manufacturing high carbon steel strip by heating a hot-rolled steel strip into a dual-phase region of ferrite-austenite at a specified heating rate, followed by annealing the steel strip at a specified cooling rate. According to the technology, the high carbon steel strip is annealed in a dual-phase region of ferrite-austenite at Ac1 point or higher temperature, thus obtaining a structure of homogeneously distributing large spheroidized cementite in the ferrite matrix. In detail, a high carbon steel containing 0.2 to 0.8% C, 0.03 to 0.30% Si, 0.20 to 1.50% Mn, 0.01 to 0.10% Sol.Al 0.0020 to 0.0100% N, and 5 to 10 Sol.Al/N is hot-rolled, pickled, and descaled, and then the descaled high carbon steel is annealed in a furnace having an atmosphere of 95% or more by volume of hydrogen and balance of nitrogen at a temperature of 680° C. or above, with a heating rate Tv (° C./hr) from 500×(0.01−N(%) as AN) to 2000×(0.1−N(%) as MN), and a soaking temperature TA(° C.) from Ac1 point to 222×C(%)2−411×C(%)+912, for a soaking time of 1 to 20 hours, followed by cooling the steel to room temperature at a cooling rate of 100° C./hr or less.
For the improvement of stretch flangeability of the high carbon steel sheet, several technologies have been studied. For example, Japanese Patent Laid-Open No. 11-269552 proposes a method for manufacturing medium to high carbon steel sheets having excellent stretch flangeability using a process containing cold rolling. According to the technology, a hot-rolled steel sheet containing 0.1 to 0.8% C by mass, and having the metal structure of substantially ferrite and pearlite, and specifying, at need, the area percentage of ferrite and the gap between pearlite lamellae, is subjected to cold rolling of 15% or more of reduction in thickness, followed by applying three-stage or two-stage annealing.
Japanese Patent Laid-Open No. 11-269553 discloses a technology of annealing a hot-rolled steel sheet containing 0.1 to 0.8% C by mass, and having a ferrite and pearlite structure with the area percentage of ferrite (%) of at or higher than a certain value determined by the C content, while applying heating and holding in the first stage and those in the second stage continuously.
Above-disclosed technologies, however, have the following-described problems.
The technology described in Japanese Patent Laid-Open No. 9-157758 anneals a high carbon steel strip in a dual phase region of ferrite-austenite at Ac1 point or higher temperature, thus forming large spheroidized cementite. It is, however, known that the coarse cementite acts as the origin of void during working step and deteriorates the hardenability owing to the slow dissolution rate of the coarse cementite. Furthermore, for the hardness after annealing, an S35C material gives Hv of 132 to 141 (HRB of 72 to 75), which cannot be said “the mild steel.”
The technologies described in Japanese Patent Laid-Open Nos. 11-269552 and 11-269553 have the ferrite structure formed by ferrite, and the ferrite contains substantially no carbide, thus the material is mild and gives high ductility. However, the stretch flangeability thereof is not necessarily favorable because the punching induces deformation at the ferrite portion in the vicinity of punched edge face so that the deformation considerably differs between the ferrite and the ferrite containing spheroidized carbide. As a result, stress intensifies in the vicinity of boundary of grains giving considerably large difference in the deformation, which results in generation of void. The void grows to crack, thus presumably deteriorating the stretch flangeability.
A countermeasure to the problem is to strengthen the spheroidizing annealing to soften the entire material. In that case, however, the spheroidized carbide becomes coarse to become the origin of void, and the carbide hardly dissolves in the heat treatment step after working, which decreases the quench strength.
Furthermore, the requirements of working level have become severer than ever from the point of productivity improvement. Accordingly, also the bore expanding working of high carbon steel sheet has become likely induced cracks on the punched edge face owing to the increase in the working degrees and other working variables. Therefore, the high carbon steel sheets are also requested to have high stretch flangeability.
Responding to those situations, we developed the technology described in Japanese Patent Laid-Open No. 2003-13145 to provide a high carbon steel sheet which hardly induces cracks on the punched edge face and which has excellent stretch flangeability. Owing to the technology, the manufacture of high carbon hot-rolled steel sheets having excellent stretch flangeability has become available.
Japanese Patent Laid-Open No. 2003-13145 is a technology of hot-rolling a steel containing 0.2 to 0.7% C by mass at a finishing temperature of (Ar3 transformation point −20° C.) or above, and cooling the hot-rolled steel sheet to a cooling-stop temperature of 650° C. or below at a cooling rate of higher than 120° C./sec, then coiling the cooled steel sheet at 600° C. or lower temperature, followed by pickling, and finally annealing the pickled steel sheet at a temperature ranging from 640° C. to Ad transformation point. As for the metal structure, the technology controls a mean diameter of carbide to a range from 0.1 μm to smaller than 1.2 μm, and the volume percentage of ferrite grains not containing carbide to 10% or less.
To reduce the manufacturing cost of driving-system parts, integral molding method using a press machine has recently been brought into practical applications. With the movement, the steel sheets as the base material are subjected to forming with combinations of complex forming modes of not only burring but also stretching, bending, and the like, thus the steel sheets are requested to have both the excellent stretch flangeability and the excellent ductility. In this regard, the technology of Japanese Patent Laid-Open No. 2003-13145 does not describe the ductility.
It could therefore be helpful to provide an ultra soft high carbon hot-rolled steel sheet which can be manufactured without applying time-consuming multi-stage annealing, which generates very few cracks on a punched edge face, and which generates very few cracks caused by press molding and cold forging, or having excellent workability giving 70% or larger hole expanding ratio λ, and 35% or larger total elongation as an evaluation index of ductility, and to provide a method for manufacturing the ultra soft high carbon hot-rolled steel sheet.